A plasma processing is widely applied in a semiconductor device manufacturing apparatus, e.g., a surface processing apparatus, an etching process or the like. In order to perform the plasma processing, a substrate mounting device for mounting a target substrate, e.g., a wafer or the like, is installed inside a processing chamber. For example, as shown in FIG. 6, the substrate mounting device includes a mounting table for mounting thereon a wafer 1 and a focus ring 3 disposed around an outer peripheral portion of the mounting table 2.
In order to perform the plasma processing on the wafer 1, the wafer 1 is mounted and fixed on the mounting table 2 while maintaining the processing chamber at a specific vacuum level. Next, a high frequency voltage is applied to the mounting table 2 to thereby generate a plasma inside the processing chamber.
Here, the focus ring 3 is installed for uniformly performing the plasma processing on the entire surface of the target substrate by moderating discontinuity of the plasma in a peripheral portion of the target substrate. To do so, the focus ring 3 is made of a conductive material and, also, a height of a top surface thereof is made substantially the same as that of a processing surface of the target substrate. Accordingly, ions are projected perpendicularly to the surface of the target substrate even in the peripheral portion of the target substrate, thereby preventing any difference from being developed between the ion density in the peripheral portion of the target substrate and that in a central portion of the target substrate. As a consequence, however, the target substrate and the focus ring 3 have substantially a same potential, and the plasma can easily flow into a backside of an end portion of the target substrate due to an electric field generated by the same potential, causing a deposition of CF-based polymer or the like onto a backside of a circumferential peripheral portion (edge portion) of the target substrate.
Therefore, Patent Document 1 suggests a technique in which the electric field in the circumferential peripheral portion (edge portion) of the target substrate is changed by adjusting a part of characteristics (impedance characteristics) of a material forming the ring so that the deposition of CF-based polymer or the like is prevented from being deposited onto the backside of the circumferential peripheral portion (edge portion) of the target substrate by inhibiting the plasma from flowing into the backside of the target substrate.
Namely, in the above technique, the focus ring 3 includes an upper member 3a and a lower member 3b, as shown in FIG. 7, and a dielectric constant thereof is adjusted by using a conductive material for the upper member 3a and a dielectric material for the lower member 3b. Accordingly, directions of electric force lines in the circumferential peripheral portion of the target substrate are changed such that the plasma is prevented from flowing into the backside of the target substrate.
Moreover, the temperature control of the wafer 1 is extremely crucial in the plasma processing, so that the wafer 1 is regulated to be maintained at a required temperature by a cooling mechanism provided in the mounting table 2. For example, a helium gas having a good thermal conductivity flows from the top surface of the mounting table 2 toward the backside of the wafer 1, thereby increasing thermal conductivity between the wafer 1 and the mounting table 2.
Besides, a heat insulating vacuum layer formed between the mounting table 2 and the focus ring 3 makes the thermal conductivity between the mounting table 2 and the focus ring 3 extremely poor in a depressurized accommodation chamber. Therefore, the focus ring 3 is not cooled at all, thus increasing the temperature of the focus ring 3. Accordingly, a composition ratio or a density of ions and radicals in the plasma in the circumferential peripheral portion of the wafer is changed. As a result, a hole penetration property (characteristic allowing the substrate to be etched to a specific depth) or an etching rate in the circumferential peripheral portion of the wafer deteriorates, or an aspect ratio of the etching deteriorates. Namely, the etching characteristics in the circumferential peripheral portion of the wafer become poor.
In order to prevent the temperature increase of the focus ring, Patent Document 2 suggests “a substrate mounting device in which a heat insulating vacuum layer is not formed by interposing a heat transfer medium between a mounting table and a focus ring and by installing a pressing unit for pressing and fixing the focus ring on the mounting table”.
Moreover, Patent Document 3 discloses therein “a substrate mounting device having an electrostatic attracting unit for attracting a focus ring”. Since, however, this device needs an electrode for electrostatically attracting the focus ring, a structure of the device becomes complicated, and the installation cost increases. In addition, the electrode for electrostatic adsorption may cause an abnormal discharge of a plasma, or generate particles or deposits.
Therefore, the inventors of the present invention suggested a unit for enhancing thermal conductivity in a peripheral portion of a wafer by interposing a heat transfer medium (corresponding to a thermally conductive sheet in the present invention) made of a gel type material between a focus ring and a mounting table of a target substrate in a plasma processing chamber. The thermally conductive sheet is made of a gel type polymer and thus has good flexibility. Accordingly, a heat insulating vacuum layer is not formed on a contact surface between the focus ring 3 and the mounting table 2, thereby facilitating the heat transfer therebetween.
Although it is preferable that the thermally conductive sheet has flexibility and thermal conductivity, the thermally conductive sheet is made of a material in which a ratio (H/k) of a hardness H of the corresponding sheet which is indicated in terms of ASKER C to a thermal conductivity k indicated as W/m·k is smaller than 20. Namely, it is important to use a material having a high thermal conductivity. Further, it has been confirmed that the presence of the thermally conductive sheet allows the focus ring 3 to be sufficiently cooled and prevents the deterioration of the etching characteristics in the circumferential peripheral portion of the target substrate.
Patent Document 1: Japanese Patent Laid-open Application No. 2005-277369
Patent Document 2: Japanese Patent Laid-open Application No. 2002-16126
Patent Document 3: Japanese Patent Laid-open Application No. 2002-33376
The above-described thermally conductive sheet is very useful to facilitate the heat transfer between the focus ring and the mounting table. However, the thermally conductive sheet is made of a gel type polymer and thus can easily adhere to a ceramic surface or a metal surface that contacts therewith. Moreover, the focus ring needs to be frequently exchanged by processes, and it is often found during the exchange procedure that the thermally conductive sheet is strongly adhered to a surface of the mounting table or the focus ring. The strongly adhered thermally conductive sheet cannot be separated if an exclusive peeling jig is not used, thus significantly deteriorating efficiency of the exchange procedure of the focus ring or the electrostatic chuck.